How to make screw spiral down into a point?
Curve Modifier. Proportional Edit. Shape Keys.



Image above 


Yellow Cylinder. High Density of Vertices is shown with the wire mesh detail on extreme left.
Blue elongated cone mesh created with proportional edit of Yellow. Vanishing Point on End. Shape Keys.
Spiral Curve and Blue Mesh with Objects centers in the same location.
Curve Modifier Shown. With the cone shape dominant.
Curve Modifier Shown. With Cylinder shape dominant.
Apply Scale to Curve.
Please research further for the Curve Modifier. This is just to launch your further research and not a tutorial.  Search here at BSE.